J. Chem. Eng. Data 1995,40, 611-614 611

Surface Tension of Alcohol + Water from 20 to 50 "C Gonzalo Vhquez,* Estrella Alvarez, and Jose M. Navaza

Department of Chemical Engineering, University of Santiago de Compostela, 15706 Santiago, Spain

The surface tension of aqueous solutions of methanol, ethanol, 1-propanol, and 2-propanol was measured over the entire concentration range at temperatures of 20-50 "C. The experimental values were correlated with temperature and with mole fraction. The maximum deviation was in both cases always less than 3%.

Introduction

The surface tension of mixtures is a physical property of great importance for mass transfer processes such as distillation, extraction, or absorption. In relation to our research (Vazquez et al., 1990) on how mass transfer is influenced by the Marangoni effect (spontaneous interfacial turbulence generated by surface tension gradients), in this work we measured the surface tension of methanol + water, ethanol + water, 1-propanol + water, and 2-pro- panol + water over the entire concentration range a t temperatures of 20-50 "C.

Experimental Section

Aqueous solutions of methanol, ethanol, 1-propanol, and 2-propanol were prepared with distilled, deionized water. The alcohols were Merck products of nominal purity >99.7%. All the solutions were prepared by weight with

Table 3. Surface Tension of 1-Propanol (1) + Water (2)

mass% XI 20 25 30 35 40 45 50 0 5

10 15 20 25 30 40 50 60 70 80 90

100

0.000 72.75 72.01 71.21 70.42 69.52 68.84 67.92 0.016 42.51 41.83 41.16 40.53 39.86 39.22 38.54 0.032 34.86 34.32 33.81 33.26 32.69 32.08 31.48 0.050 30.87 30.36 29.88 29.39 28.89 28.36 27.90 0.070 28.31 27.84 27.41 26.96 26.51 26.03 25.59 0.091 27.08 26.64 26.22 25.79 25.36 24.91 24.49 0.114 26.41 25.98 25.56 25.16 24.74 24.29 23.88 0.167 25.68 25.26 24.88 24.51 24.09 23.69 23.32 0.231 25.18 24.80 24.42 24.02 23.64 23.24 22.86 0.310 24.89 24.49 24.11 23.73 23.33 22.92 22.54 0.412 24.47 24.08 23.69 23.31 22.93 22.54 22.14 0.545 24.23 23.86 23.48 23.09 22.68 22.28 21.91 0.730 23.98 23.59 23.21 22.84 22.44 22.04 21.66 1.000 23.69 23.28 22.89 22.51 22.11 21.69 21.31

Table 4. Surface Tension of 2-Propanol (1) + Water (2)

Table 1. Surface Tension of Methanol (1) + Water (2) mass% x1 20 25 30 35 40 45 50

mass % x1 20 25 30 35 40 45 50 0 5

10 15 20 25 30 40 50 60 70 80 90

100

0.000 72.75 72.01 71.21 70.42 69.52 68.84 67.92 0.029 63.46 62.77 61.98 61.14 60.32 59.58 58.77 0.059 56.87 56.18 55.41 54.67 54.01 53.27 52.46 0.090 51.83 51.17 50.43 49.76 47.04 48.39 47.62 0.123 47.86 47.21 46.56 45.84 45.17 44.48 43.76 0.158 44.38 43.78 43.14 42.51 41.82 41.21 40.57 0.194 41.67 41.09 40.43 39.77 39.14 38.53 37.88 0.273 37.02 36.51 35.90 35.36 34.79 34.18 33.62 0.360 33.37 32.86 32.33 31.85 31.26 30.77 30.28 0.458 30.32 29.83 29.34 28.86 28.44 27.93 27.54 0.568 27.91 27.48 26.99 26.56 26.12 25.64 25.23 0.692 25.98 25.54 25.06 24.60 24.21 23.72 23.33 0.835 24.37 23.93 23.43 22.95 22.57 22.06 21.67 1.000 22.95 22.51 22.01 21.52 21.13 20.61 20.21

Table 2. Surface Tension of Ethanol (1) + Water (2)

mass % 0 5

10 15 20 25 30 40 50 60 70 80 90

100

~

x1

0.000 0.020 0.042 0.065 0.089 0.115 0.144 0.207 0.281 0.570 0.477 0.610 0.779 1.000

~ ~ ~~

u/(mN.m-') a t t/"C 20 25 30 35 40 45 50

72.75 72.01 71.21 70.42 69.52 68.84 67.92 56.41 55.73 55.04 54.36 53.63 52.96 52.16 48.14 47.53 46.88 46.24 45.58 44.97 44.26 42.72 42.08 41.49 40.88 40.27 39.64 38.96 38.56 37.97 37.38 36.83 36.28 35.71 35.12 36.09 35.51 34.96 34.41 33.86 33.31 32.76 33.53 32.98 32.43 31.94 31.42 30.89 30.34 30.69 30.16 29.68 29.27 28.77 28.28 27.82 28.51 27.96 27.53 27.11 26.64 26.21 25.78 26.72 26.23 25.81 25.43 24.97 24.54 24.11 25.48 25.01 24.60 24.21 23.76 23.33 22.92 24.32 23.82 23.39 23.01 22.54 22.12 21.71 23.23 22.72 22.32 21.94 21.53 21.13 20.71 22.31 21.82 21.41 21.04 20.62 20.22 19.82

0021-9568/95/1740-0611$09.00/0 0

0 5

10 15 20 25 30 40 50 60 70 80 90

100

0.000 72.75 72.01 71.21 70.42 69.52 68.84 67.92 0.016 50.32 49.58 48.88 48.16 47.37 46.66 45.82 0.032 41.21 40.42 39.73 39.06 38.43 37.78 37.04 0.050 35.27 34.63 34.01 33.38 32.76 32.13 31.51 0.070 31.16 30.57 29.98 29.37 28.79 28.18 27.59 0.091 28.88 28.28 27.71 27.14 26.58 26.04 25.47 0.114 27.38 26.82 26.26 25.73 25.18 24.66 24.11 0.167 25.81 25.27 24.74 24.23 23.72 23.21 22.69 0.231 24.78 24.26 23.76 23.27 22.78 22.29 21.81 0.310 24.05 23.51 22.97 22.54 22.03 21.52 21.01 0.412 23.17 22.68 22.18 21.71 21.22 20.76 20.28 0.545 22.62 22.14 21.66 21.18 20.71 20.23 19.78 0.730 22.21 21.69 21.18 20.66 20.16 19.74 19.23 1.000 21.74 21.22 20.72 20.23 19.71 19.21 18.69

deviations of less than 0.3% from the desired concentra- tions, which are indicated in Tables 1-4 as mole fraction.

Surface tension was determined a t 5 "C intervals be- tween 20 and 50 "C, using a Prolabo tensiometer, which employs the Wilhelmy plate principle (Lin et al., 1990; Bogaert et al., 1980), thermostated with a precision of f O . l "C. Values shown below are averages of 5-10 measure- ments; maximum deviations from the average were always less than 0.4%.

Results and Discussion Tables 1-4 list the measured surface tensions of metha-

nol + water, ethanol + water, 1-propanol + water, and 2-propanol + water of various concentrations a t each temperature. In all the systems studied surface tension, u, decreased with increasing temperature for any given mole fraction of alcohol. The surface tensions of the pure components can be correlated with temperature by fitting

1995 American Chemical Society

612 Journal of Chemical and Engineering Data, Vol. 40, No. 3, 1995

- 70- 6

z

7

e

v \

b so

30

-

-

10 1s 25 35 45 55

Figure 1. Surface tension as a function of temperature: 0, water; 0 , 5 mass % ethanol; D, 10 mass % ethanol; 0 ,20 mass % ethanol; A, 50 mass 70 ethanol; V, 80 mass % ethanol; 0, 100 mass 9%

t I "C

ethanol; -, eq 1.

1

0.8

Y b

0.6

0.4

0.1

I 0.2 0.4 0.6 0.8 1

X1

Figure 2. Dimensionless surface tension o* plotted against the mole fraction of alcohol: 0, methanol 20 "C; 0, methanol, 50 "C; m, ethanol, 20 "C; 0, ethanol, 50 "C; V, 1-propanol, 20 "C; A, 1-propanol, 50 "C; 0, 2-propanol, 20 "C; V, 2-propanol, 50 "C. the following linear expression (Jasper, 1972):

o/(mN.m-') = ICl - K,(t/"C) (1) Equation 1 also fitted the data of Tables 1-4 for each

concentration, with deviations of less than 1%. Figure 1

1

a

0.9

0.8 15 25 35 45 55

t I "C Figure 3. Fitted parameter a (eq 2) as a function of tempera- ture: D, methanol; A, ethanol; 0, 1-propanol; V, 2-propanol.

1

b

0.95

0.9

L

A 0.85 ' I I I

15 25 35 45 55

t I O C Figure 4. Fitted parameter b (eq 2) as a function of tempera- ture: D, methanol; A, ethanol; 0, 1-propanol; V, 2-propanol.

shows the results for ethanol + water as an example. The fitted values of KI and K2 are listed in Table 5 .

For a given temperature, the surface tension of mixtures studied in this paper decreased as the alcohol concentration increased. This trend was nonlinear, the change in surface tension caused by a given change in alcohol concentration being larger at low concentrations than a t high concentra-

Journal of Chemical and Engineering Data, Vol. 40, No. 3, 1995 613

I

B

0 0.2 0.4 0.6 0.8 1

X1

Figure 5. Surface tension of methanol + water plotted against the mole fraction of methanol: 0, exptl, 20 "C; -, calcd, 20 "C; A, exptl, 30 "C; - * a , 30 "C; V, exptl, 40 "C; - - -, calcd, 40 "C; 0, exptl, 50 "C; -, calcd, 50 "C.

Table 5. Surface Tension Parameters KI and KZ for Aqueous Organic Mixtures

organic component x i Ki Kz xi Ki K2 methanol 0.000 76.0114 0.1609 0.273 39.3594 0.1149

0.029 66.6950 0.1584 0.360 35.4515 0.1041 0.059 59.7657 0.1457 0.458 32.2614 0.0963 0.090 54.6635 0.1407 0.568 29.8386 0.0938 0.123 50.6179 0.1379 0.692 27.8830 0.0926 0.158 47.0057 0.1289 0.835 26.2241 0.0921 0.194 44.2404 0.1276 1.000 24.7789 0.0918

ethanol 0.000 76.0114 0.1609 0.207 32.5586 0.0949 0.020 59.2521 0.1407 0.281 30.2507 0.0899 0.042 50.7436 0.1290 0.370 28.4139 0.0861 0.065 45.2079 0.1241 0.477 27.1571 0.0849 0.089 40.8207 0.1139 0.610 25.9242 0.0834 0.115 38.2868 0.1106 0.779 24.8225 0.0824 0.114 35.6229 0.1054 1.000 23.8993 0.0819

1-propanol 0.000 76.0114 0.1609 0.167 27.4226 0.0831 0.016 45.1418 0.1319 0.231 26.8879 0.0821 0.032 37.0182 0.1092 0.310 26.5612 0.0812 0.050 32.8536 0.0993 0.412 26.1232 0.0804 0.070 30.1200 0.0906 0.545 25.8657 0.0798 0.091 28.8068 0.0864 0.730 25.5904 0.0794 0.114 28.0933 0.0844 1.000 25.2721 0.0791

2-propanol 0.000 76.0114 0.1609 0.167 27.8879 0.1039 0.016 53.7768 0.1469 0.231 26.8821 0.1024 0.032 44.1600 0.1314 0.310 26.0786 0.1014 0.050 37.7668 0.1252 0.412 25.2489 0.1005 0.070 33.5471 0.1191 0.545 24.6712 0.0998 0.091 31.1171 0.1131 0.730 24.1807 0.0993 0.114 29.5368 0.1086 1.000 23.6891 0.0989

tions. Fitting the equat ion

a,- u 1 + a x , a*=--- - a, - a. 1 - bx,"'

t o the d a t a for each solute (where a, a n d a,, are the surface tensions of pure w a t e r a n d p u r e alcohol, respectively, a n d x1 a n d x2 are the mole fractions of alcohol a n d water) yielded 8 - x1 curves like those shown in Figure 2 for

40 n * k 0

35 E

b

v \

30

25

20

15

lb \ ' . d

0 092 094 096 098 1

X1

Figure 6. Surface tension of 2-propanol + water against the mole fraction of 2-propanol: ., Hoke et al., 1991,25 "C; A, Hoke et al., 1991,40 "C; V, Hoke et al., 1991,50 "C; 0, Cheong et al., 1987,25 "C; -, eq 2, 25 "C; . . e , eq 2, 40 "C; - - -, eq 2, 50 "C.

0 092 094 096 098 1

X1

Figure 7. Excess surface tension uE as a function of the mole fraction of alcohol: 0, methanol, 20 "C; +, methanol, 50 "C; A, ethanol, 20 "C; 0, ethanol, 50 "C; 0, 1-propanol, 20 "C; V, 1-propanol, 50 "C; - calcd from eqs 2 and 3.

t empera tures of 20 and 50 "C. The dimensionless surface tension 8 can be considered as independent of tempera ture over most of the concentration range. The values of the f i t ted parameters a and b in eq 2 are linear functions of tempera ture for each alcohol (see Figures 3 a n d 4). Figure

614 Journal of Chemical and Engineering Data, Vol. 40, No. 3, 1995

5 shows, by way of example, the measured surface tensions of methanol + water in comparison with those calculated by means of eq 2. The deviations are less than 3% in all such plots.

Experimental values obtained by other researchers can- not be directly compared with the values reported in this paper because of differences in the concentrations used. They can, however, be compared with values predicted by eq 2. In the previous reports considered (for methanol + water a t 25 "C (Cheong and Carr, 19871, ethanol + water a t 25 "C (Ernst et al., 1936), 1-propanol + water at 20, 30, 40, and 50 "C (Martin et al., 19831, and 2-propanol + water at 25 "C (Cheong and Carr, 1987; Hoke et al., 1991) and a t 30, 40, and 50 "C (Hoke et al., 1991)), 92% of the values deviate by less than 6% from the predictions of eq 2. The results for 2-propanol + water are shown as an example in Figure 6.

The sensitivity of the surface tension of these systems to the changes in concentration in the low alcohol concen- tration region reflects the nonideal character of these mixtures. The deviation from ideal behavior can be quantified by the excess surface tension aE, defined by

2 = c7 - (og, + O&) (3) Plotting uE against the mole fraction of organic compo-

nent (Figure 7 shows typical plots) shows that deviation from ideality increases with the length of the organic molecule and decreases with rising temperature.

Literature Cited Bogaert, R.; Joos, P. Diffusion-Controled Adsorption Kinetics for a

Mixture of Surface Active Agents at The Solution-Air Interface. J. Phys. Chem. 1980,84, 190-194.

Cheong, W. J.; Carr, P. W. The Surface Tension of Mixtures of Methanol, Acetonitrile, Tetrahydrofuran, Isopropanol, Tertiary Bu- tanol and Dimethylsulfoxide with Water a t 25 "C. J. Liq. Chro- matogr. 1987, 10, 561-581.

Emst. R. C.: Watkins. C. H.: Ruwe. H. H. The Phvsical ProDerties of The Tern& System Ethyl Alcohol-Glycerin-Water. J. Phis. Chem. 1936,40,627-635.

Hoke, B. C.; Chen, J. C. Binary Aqueous-Organic Surface Tension Temperature Dependence. J. Chem. Eng. Data 1991,36,322-326.

Jasper, J. J. Surface Tension of Pure Liquid Compounds. J. Phys. Chem. Ref. Data 1972,1, 841-1009.

Lin, S.; McKeigue, K.; Maldarelli, C. Diffusion-Controled Surfactant Adsorption Studied by Pendant Drop Digitalization. AIChE J. 1990, 36, 1785-1793.

Martin, A.; Rosell6, A.; Rodriguez, J. M.; Hormigo, A. Estimation of Surface Tensions of Mixtures at Their Boiling Point. Ing. Quim. 1983,166, 127-132.

Vbzquez, G.; Antorrena, G.; Navaza, J. M. Estimation of The Turbu- lence Induced by The Marangoni Effect at a Gas-Liquid Interface. Int. Chem. Eng. 1990,30, 228-235.

Received for review September 23, 1994. Accepted January 10, 1995.@ This work was partially financed by the Xunta de Galicia (Spain) (Grant XUGA 20909 B94).

JE940199T

@ Abstract published in Advance ACS Abstracts, March 1, 1995.